POSTNATAL CHANGES IN MITOCHONDRIAL PROTEIN MASS AND RESPIRATION IN SKELETAL-MUSCLE FROM THE NEWBORN PIG

Quantitative and functional changes occurring in mitochondria were stu died in pig skeletal muscle between birth and 5 days of life. Postnata l changes were followed separately on intermyofibrillar and subsarcole mmal mitochondria isolated from rhomboideus (RH) and longissimus dorsi (LD) muscles. The integrity and purity of the isolated mitochondria w as checked by electron microscopic observations. The mass of mitochond rial protein was not different between muscles at birth. It increased tremendously during the first 5 days of life, by 49% in LD (P < 0.001) and 93% in RH (P < 0.001) muscle and was 30% higher in RH than in LD muscle at 5 days of life (P < 0.05). Mitochondria isolated from RH mus cle exhibited 30% higher oxidative and phosphorylative capacities than those from LD muscle at 5 days of life (P < 0.05). Intermyofibrillar (IM) mitochondria had high respiration rate, enzyme activities and cou pling parameters (respiratory control ratio, phosphorus-oxygen ratio) from birth. Subsarcolemmal (SS) mitochondria were less active than IM mitochondria; their respiration rate and enzyme activities were 60% lo wer (P < 0.01) and increased with age, particularly in LD muscle (P < 0.05). Short-term cold exposure had no effect on mitochondrial mass an d activity. These results suggest that muscle mitochondria are functio nal from birth and are changing primarily quantitatively. SS and IM mi tochondria exhibit specific changes that are probably involved in the postnatal acquisition of skeletal muscle oxidative metabolism. (C) 199 7 Elsevier Science Inc.